J.G.
While I wouldn't feel weird bringing $5 for pizza, I would feel weird being asked, esp if I am bringing a gift to the kiddo (which I would prefer to do. I love shopping for kids!)
So yes, either make a lot of chips and dip and dessert. Or maybe just get some hot dogs, keeping them warm in a crock pot or something. Hot dogs are cheap, as are buns and other toppings (relish, mustard, ketchup, chili). Make tea (or is this too Texan?) and have water avail. Lemonade maybe too. Don't waste money on sodas or alcoholic beverages. Somewhere in the invite you could put BYOB or "lemonade and tea will be served."
As for cakes/cupcakes. Cupcakes you would not need to get an extra plate for. It'd kind of be in it's own plate (the paper). BUT people may eat two each instead of one each - where with a cake, I normally get one slice and that's it. I don't think it would make a difference either way with the actual cost of the cake itself.